The correct tire pressure in the wheels of a motor vehicle is an essential safety factor for their operation during driving, and it must constantly be ensured that it is always kept within the tolerance limits. The tire pressure of a motor vehicle influences not only, to a particular extent, safety during braking and steering, but also the useful life of the tire as well as the driving behavior of the vehicle. Furthermore, by a deliberate variation of the tire pressure, the driving properties of a vehicle can be set optimally for travel on paved roads and in terrain.
In conventional tire pressure regulating systems, for example according to U.S. Pat. No. 4,434,833, compressed air is supplied from an air supply source via an air pipe to an air passage in an axle housing, and the compressed air is subsequently fed into the tire via a further air passage and an air pipe in the axle.
DE 28 54 772 A1 shows a device for monitoring the tire pressure on a motor vehicle, in which a pressure testing system is installed stationarily for each tire and consists in each case of a pressure tester connected to a valve of the tire via an air line. This pressure tester is connected via an electrical line to an electronic monitoring apparatus which responds to pressure and to which a signal generator is assigned.
Since the supply of air for a vehicle tire in tire pressure regulating systems usually takes place through the wheel bearing designed as a rolling bearing, an essential aspect is the gas tightness of the gas supply particularly in the rolling bearing or in its immediate surroundings. In this case, a compressed air leakage as a result of the penetration of leakage air from the sealing system of the compressed air leadthrough into the rolling bearing is to be avoided.
Known seals on compressed air leadthroughs through a wheel bearing nevertheless have a more or less pronounced leakage. On account of this, contaminated air passes in an undesirable way into the inner space of the rolling bearing, and this may lead to the contamination of the lubricant, for example the bearing grease. Furthermore, water may penetrate into the rolling bearing and cause deficient lubrication. Solid particles penetrating into the wheel bearing are rolled over there by the rolling bodies, and therefore imprints of material in the running surfaces occur. If the internal pressure in the rolling bearing rises, this leads to higher seal wear and to the failure of the seal. Moreover, the lubricant is pressed out of the rolling bearing, and this likewise causes deficient lubrication. Sealing lips present in the rolling bearing are also often overturned or seals are pressed out of the rolling bearing. All these problems lead to premature bearing failure and therefore to increased costs. A high leaktightness, reliable throughout the desired useful life, of the rolling bearing with respect to the devices of the tire pressure regulating system is therefore of particular interest.
DE 199 16 106 C2 discloses a machine element as a leadthrough element for leading through media during rotation of two machine parts which are guided with respect to one another on rolling bearings and through which a medium flows and between which a seal is arranged. The seal possesses two pairs of sealing lips, one pair of sealing lips of which is located in one machine part and seals off dynamically with respect to this. The second pair of sealing lips is seated firmly in the second machine part and seals off statically with respect to this. Between the pairs of sealing lips are located bores which serve for the throughflow of media, such as, for example, gas, compressed air, oil, water, etc. Furthermore, spring elements are mounted between the pairs of sealing lips and ensure that the pairs of sealing lips are pressed firmly against the machine parts. A disadvantage in this case is the complicated set-up of the seal due to the provision of the spring elements and the large number of sealing lips in order to achieve leaktightness between the machine elements and the rolling bearing.
DE 690 05 731 T2 discloses a sealing device for a tire pressure regulating system, in which a rolling bearing is arranged between an axle housing guided on an axle and a hub carrying a wheel disk with a tire. This rolling bearing is located in an air conduit consisting of air pipes and has centrally an annular seal device which is fastened firmly to the inner circumferential surface of the hub. The seal device consists of a plurality of stepped parts, angled parts, holding rings, spacer rings and end plates with main and auxiliary seals consisting of fluororubber and having a plurality of sealing lips which bear against the axle housing. The main seal has a self-lubricating thin plate which is fastened to the lip sliding surface and which is in sliding contact with the axle housing. The seal device has a highly complex and complicated set-up and requires a considerably large construction space. Furthermore, the useful life and therefore the leaktightness of the seal device are limited particularly because of the thin plate on the lip sliding surface.
A tire pressure regulating device according to EP 0 224 674 B1 possesses an axial compressed air supply through the wheel hub into an annular space between the wheel hub and the wheel hub housing. The annular space is sealed with respect to the lubricant-carrying spaces by means of seals. The seal device consists of multiple labyrinth seals which are framed in a sleeve located on the wheel hub side and in a sleeve located on the wheel hub housing side. The seal lamellae consist of steel coated with plastic flakes and grease being introduced into the lamellae interspaces for sealing-off purposes. The labyrinth seals are therefore complicated to produce and to mount.